The present invention relates to a method for performing surface treatment of an aluminum plate used for a printing plate, and particularly relates to a method for recovering an etching liquid containing sodium hydroxide as a main component thereof and a method for improving the quality of aluminum slag generated in the surface treatment.
Various treatment liquids are used in performing surface treatment of an aluminum plate for a printing plate. For example, nitric acid and hydrochloric acid are used in an electrical surface-roughening process, sulfuric acid is used in an anodizing process, and aluminum hydroxide is generally used in a process of etching aluminum chemically.
On the other hand, as a support or substrate for a lithographic press plate, an aluminum plate or an aluminum alloy plate is used, and the surface shape or the like of the substrate after being treated varies largely depending on the concentration of aluminum ions in the above-mentioned treatment liquid, so that the aluminum ions greatly affect the quality of a plate used for a printing plate.
For example, when electrochemically roughening an aluminum plate with nitric acid, the concentration of nitric acid is preferably selected to have a value ranging from 5 grams/liter to 30 grams/liter to perform uniform roughening, and the concentration of the aluminum ions is preferably selected to have a value ranging approximately from 5 grams/liter to 15 grams/liter.
Additionally, when performing an anodizing treatment with sulfuric acid, the concentration of sulfuric acid preferably ranges from 50 grams/liter to 300 grams/liter. If the concentration of the aluminum ions exceeds 15 grams/liter, an anodized film cannot be produced uniformly on the aluminum plate surface.
Generally, an etching treatment using sodium hydroxide is performed as a process after roughening the surface mechanically or before or after roughening the surface with nitric acid or sulfuric acid electrochemically. In most cases, aluminum ions exist in a treatment liquid. Thus, the optimum values of the respective concentrations of the sodium hydroxide and aluminum ions range from 200 to 600 grams/liter, which is not more than the saturation concentration, and from 10 to 100 grams/liter. Additionally, the ratio of the aluminum ion concentration to the sodium hydroxide concentration is low, and generally the concentration of aluminum ions is made no higher than the saturation concentration.
As described above, the aluminum ion concentration greatly influences the quality of an aluminum substrate used for a printing plate, and hence maintaining the proper aluminum ion concentration in the treatment liquid to a predetermined value is critical. To maintain the concentration of aluminum constant, the treatment liquid in the treatment system has generally been discharged externally of the system, or, as disclosed in Japanese Unexamined Patent Publication No. Sho-57-192300, the aluminum ions in a treatment liquid have been absorbed by an ion-exchange resin.
However, in the method for discharging the treatment liquid in a treatment system externally of the system to adjust the concentration of aluminum ions, treatment liquid must be discharged together with the aluminum ions, so that the quantity of the treatment liquid used becomes very large and the amount of waste treatment liquid is also large.
In the method for absorbing aluminum ions with an ion-exchange resin to discharge the aluminum ions externally of a system, the ratio of removing the aluminum ions relative to the amount of ion-exchange resin used is so low that a large quantity of resin must be used. Furthermore, the resin must be exchanged frequently, for instance, every three months, and thus the operating costs of the system are high.
Additionally, the present applicants have earlier proposed (see Japanese Unexamined Patent Publication No. Hei-1-200992) a method of discharging aluminum ions externally of a system through diffusion dialysis using an ion-exchange resin to separate aluminum ions from sodium hydroxide. However, in diffusion dialysis, aluminum hydroxide scale and the like tends to adhere to the waste liquid side after removal of undissolved metal such as iron adhering to a film. Thus, after the diffusion dialysis, an acid such as sulfuric acid must be used to clean the system, for instance, every three days, thereby increasing the cost of using such a system. Additionally, a large amount of aluminum slag is discharged during the surface treatment process of an aluminum plate, and there has not been any commercial use of this slag, so that managing and disposing of the slag is burdensome.
Additionally, in accordance with a technique disclosed in Japanese Examined Patent Publication No. Sho-57-2649, a silicic acid is added, but applications of this technique are limited, and the technique is not preferably used for the surface treatment of a substrate for a lithographic press plate.
In view of the foregoing conventional methods, when performing surface treatment of an aluminum plate for a lithographic press plate using various alkali treatment liquids, and particularly when performing a surface etching treatment using sodium hydroxide, it has been difficult to maintain the concentration of aluminum ions in a treatment system to a predetermined value to make the quality of the treated surface superior and uniform. Additionally, it has been difficult from the standpoint of environmental integrity and operating costs to reuse the large amount of aluminum slag discharged during the surface treatment process. However, as described above, no suitable method has yet been proposed.
Another problem that the invention is directed to solving is to accelerate the crystallizing reaction of the etching liquid and the aluminum hydroxide and to reduce impurities contained in the aluminum hydroxide changed in quality when the liquid in which the above-mentioned aluminum slag is dissolved is crystallized and the aluminum slag is changed in quality.
Thus, the conventional methods and apparatus for performing surface treatment of an aluminum plate for a lithographic printing plate using various alkali treatment liquids, and particularly surface etching using sodium hydroxide, are inadequate to: maintain a predetermined concentration of aluminum ions in a treatment liquid (etching liquid) in a treatment system to make the quality of the treated surface superior and uniform, industrially reuse the aluminum hydroxide recovered from the etching liquid, and accelerate the crystallizing reaction to recover the etching liquid, and reduce impurities contained in the recovered aluminum hydroxide. However, as has been described above, no acceptable method therefor has been conventionally proposed.